The application of protective and decorative coatings in the nature of paint or varnish by electrophoretic deposition now is a large industry. Quite commonly such coating processes are called "electrocoating". U.S. Patent and Trademark Office Class 204, subclass 181, reflects the large growth of this technology in recent years.
Generally, in an electrocoating process of the type concerned here, one or more cathode workpieces and one or more counterelectrodes (anodes) are maintained for a short period of time in an electrical circuit with a dilute aqueous dispersion of film-forming paint binder, usually pigmented, between them. In most such coating operations a net unidirectional electric current is passed between these electrodes at fair voltage (e.g., 50+ V.). Generally such current is rectified ac current. This causes deposition of the coating on the electrodes (workpieces). Most frequently the binder dispersion is maintained as a bath in which the electrodes are at least partially immersed. Other methods for forming the electrical circuit also have been proposed, e.g., by showering the workpieces with continuous streams of the current-carrying aqueous coating dispersion.
Representative of the earliest practical electrocoating is that shown in Allan E. Gilchrist's U.S. Pat. No. 3,230,162 of 1966. More recently cationic or cathodic electrocoating has become popular. U.S. Pat. No. 3,799,854 and many subsequent patents involving the electrodeposition of blocked isocyanate and amino resins onto a cathodic workpiece, and the subsequent curing of the electrodeposited film into a crosslinked structure are representative of such cathodic electrocoatings. These very practical techniques employ comparatively low molecular weight, structurally ionized resinous material as their fundamental paint binders for the electrocoating, in contrast to the instant latices made by emulsion polymerization.
That is not to say that the cathodic electrodeposition of synthetic latices has not been proposed before. Representative cathodic electrocoating proposals using synthetic latex binders include those of these U.S. Pat. Nos. 3,873,488; 3,882,009; 3,994,792; 3,998,776; 4,017,372; 4,225,406; 4,225,407; and 4,379,869.
Pigmentation is added most efficiently to a cathodic electrocoating latex formulation in the form of a pigment dispersion or "grind" containing resinous pigment dispersant that has molecular weight substantially lower than that of the latex (e.g., not substantially above about 20,000 for such dispersant as compared to at least about 50,000 and usually much higher for such latex--both weight averages). We have found that the solvent resistance of the resulting cured latex-bearing film can be impaired significantly by such pigmenting technique even though the resinous pigment dispersant is in a minor proportion relative to the latex.
Synthetic cation-active latex binders have been cathodically electrodeposited by themselves ("clears") from aqueous dispersion to provide films. Some of these latices are thermoplastic, and their cured films lack solvent resistance. Others are self-crosslinking when cured, e.g., with heat; their films have good solvent resistance. However, when a small proportion of resinous material of substantially lower molecular weight, such as a resinous pigment dispersant, is incorporated into such aqueous self-crosslinking latex for codeposition, the solvent resistance of the cured latex-rich film surprisingly can be quite adversely affected (despite generalized teachings to the effect that customary pigmentation is permissible in conventional cathodic electrocoating latex compositions).
Advantages of the instant invention over prior proposed latex formulations for cathodic electrocoating include the capacity to obtain pigmented cured films having good solvent resistance. The instant formulations have the additional advantage of being able to be applied without offensive organic sulfur odor problems, a complaint made about some prior art cathodic electrocoating of latices. In other words, organic sulfur compounds need not be used in the synthesis in such a way as to give odor problems.